The most popular method of locating new oil deposits utilizes reflection and/or refraction seismic exploration. The method usually employs a vibration source on the surface of the ground which imparts seismic waves into the ground. These waves either reflect or refract from boundaries which have a change in the seismic impedance. (Seismic impedance is the velocity of sound times the density of the media.) The returning sound waves are detected by seismometers deployed on the surface of the ground. Through a complicated analysis process, the sound velocity and structure of the ground is determined. Physical properties of earth, such as its composition, can then be deduced from the velocity of sound.
This method is very popular and effective but suffers from definite, known problems:
First, this prior-art method makes it quite expensive to produce data with fine resolution.
Second, the process depends on surfaces which reflect or refract the sound waves, in order to return the signals so that they can be detected. This condition is absent in approximately one-third of the world.
Third, the top of the ground is highly weathered, and the weathering strongly attenuates the source waves and the returning waves. The effect is that the data deteriorate, due to the reduction in signal-to-noise ratio.
Fourth, most noise is generated at the surface by wind or machinery, and it is attenuated with depth. Broadcasting and receiving seismic signals below the ground's surface benefits from lower ambient noise levels.
A method which overcomes these problems involves locating the vibration source down a well and positioning geophones both on the surface of the ground and down into adjacent wells.
The down hole seismic exploration method is primarily a transmission technique, in contrast to surface methods which are based on reflection and/or refraction of seismic waves. In down-hole exploration, the waves are simply transmitted from the source (spear) to the detector (geophones). The velocity is deduced from the travel time between the transmitter and receiver, for the velocity of the sound is simply the distance traveled divided by the travel time. Geological features which do not possess a change in acoustic impedance can be defined by transmission seismic techniques because reflection or refraction techniques are not required. Indeed, most oil fields have been mapped with surface seismic techniques, and such surface techniques may have missed oil-bearing regions or changes in geologic structure which could block the flow of oil to a well. The primary use for the down-hole seismic method is in refining the definition of existing oil fields in order to assist in the placement of new wells for recovering untapped oil reserves which are in close proximity to existing wells.